The objectives of this project are: 1. To purify by selective precipitation and chromatography a folate-binding factor recently identified in chronic myelogenous leukemia cells. 2. To determine the role of the folate-binding factor on DNA-thymine synthesis using in vitro enzymic system involving reduction of dihydrofolate to tetrahydrofolate and thymidine synthesis from deoxyuridine. 3. To determine whether this folate binder can interfere with pharmacologic action of methotrexate by preventing the drug from inactivating folate reductase. 4. To purify the folate-binding factor from milk using selective precipitation and chromatography and to determine the effect of this binder on: a) in vitro folate reactions and thymidine synthesis; b) serum folate concentrations, in experimental animals; c) growth of bacterial and mammalian cells in tissue culture. 5. To study the in vivo and in vitro biologic effect of antibodies with binding determinants for various folate analogues. 6. To study the guinea pig's mechanism of resistance to methotrexate by determining whether: a) there is an active or inducible alternate pathway for de novo thymidine synthesis, and; b) whether there is a folate-binding factor in replicating tissue which might also bind methotrexate. 7. To correlate folate reductase levels with methotrexate concentrations in various human leukemia cells after treatment with various combinations of methotrexate and actinomycin-D.